engineered syntactic products

Acoustic Properties of Syntactic Foam – Part 3

Enhanced Density Products

The following measurements were made on our low density syntactic foams:

Depth Rating (meters) Average      Density (Kg/m3) Average

Speed of Sound (m/s)



1000 385 2,186 0.84
2000 400 2,461 0.99
3000 432 2,698 1.17
4000 457 2,767 1.26
4000 515 2,675 1.38
4500 490 2,922 1.43
5000 496 2,968 1.47
6000 552 2,952 1.63
7000 545 3,100 1.69

As noted, even though the hollow glass spheres and resin systems were different, the speed of sound and impedance measurements tracked well with depth rating.

It should be noted that these measurements were made on what we consider to be some of our standard buoyancy products, not materials tailored specifically to provide specific or targeted acoustic properties.  For example, formulations made to Navy specifications have the following properties:

Formulation 1

Density: 690 kg/m3 ± 16

Speed of Sound: 2,850 m/s ± 100

Acoustic Impedance: not specified

Formulation 2

Density: 380 kg/m3 ± 32

Speed of Sound: 2,595 m/s

Acoustic Impedance: 0.940 MRayls

In these instances, specific combinations of resins and microspheres were used to attain the desired properties.  This approach is always an option, though it does require some development and input from the end user.

Properties of CMT Materials Standard Tooling Products

The following measurements were made on our standard plug-assist tooling material (HYTAC®)

Product Name Avg Density (Kg/m3) Avg Speed of Sound (m/s) Impedance


HYTAC-W 690 2,151 1.48
HYTAC-B1X 708 2,568 1.82
HYTAC-C1R 747 2,579 1.93
HYTAC-XTL 755 2,697 2.04
HYTAC-FLX 838 2,749 2.30
HYTAC-WF 838 2,788 2.34
HYTAC-FLXT 897 2,647 2.38
HYTAC-WFT 968 2,700 2.61


While HYTAC materials are not optimized in any way around acoustic properties, the information is interesting nonetheless.  HYTAC-W, for example, is very close to a match for seawater with minimal inconsistencies in terms of density stratification.  As with all of the plug assist tooling products, HYTAC-W is available in a wide range of sizes and shapes.  For example, 2” diameter rods are maintained as a stock item.  This would yield an immediate ~22% savings on material for a given diameter compared to the same amount of material taken from cut blocks if the application required cylindrical shapes.  Sheet sizes starting at one-inch thick up to six-inch thick are also standard, at 0.5-inch increments.  Again, this could mean a significant material savings.

There are several additional observations of note from these test results.  Two sets of the products have derivatives that include polytetrafluoroethylene (PTFE) in syntactic.  (HYTAC-FLX => HYTAC-FLXT and HYTAC-WF => HYTAC-WFT). The PTFE is added to improve the slip at the surface of the material during the thermoforming process and to prevent sticking and material build-up (for more information on thermoforming, visit  The addition of PTFE increases the density but results in a decrease in the speed of sound.  Correspondingly, each material also shows a slight decrease in modulus.  It is also interesting to note the difference we see when making a dramatic change in the matrix material.  There is only a very subtle difference in density between HYTAC-W and HYTAC-B1X, yet the average speed of sound is significantly different due to the change from an epoxy matrix (W) to a thermoplastic matrix (B1X).

In summary, these simple observations illustrate the flexibility available to us as formulators.  The wide range of end-products allows the user to select an off-the-shelf product that may just as easily meet their needs as a complex, unique formulation.

syntactic foam

Acoustic Properties of Syntactic Foam – Part 2

Comparative Testing Procedures

Tests were done on two formulations with densities of 24 lb/ft3, the lowest density that we can currently provide for a microsphere syntactic foam. One formulation was rated for 1000 meters (MZ grade) and one was rated for 2000 meters (BZ grade). The formulations were designed for the specific depths using different resin systems and glass bubbles, so we expected to see some significant differences. The lower strength and stiffness MZ material had a speed of sound of 2,186 m/s while the BZ sample had a speed of sound of 2,489 m/s. This is interesting because the values nearly mimic the average modulus difference between the two products. The compressive modulus of BZ is about 1.15 times that of MZ. It is impossible, however, to separate whether the stiffness difference is due solely to the higher modulus resin or the different glass bubbles.

Overall we can report the following general information from the test:

Sample Density (Kg/m3) Speed of Sound (m/s) Impedance (MRayls)
MZ 385 2,186 0.84
BZ 385 2,489 0.96

The process for manufacturing low density syntactic foam does not lend itself to producing a product that is perfectly consistent in terms of density. This density difference can appear from block-to-block as well as within a block. The natural size and density variation of the hollow glass spheres that make up ~ 70% of the syntactic structure are the main reason for these differences. As made, the hollow glass spheres can vary by ±15 % in density in a single batch. This results in a density acceptance standard of ± 2 lb/ft3 (± 32 Kg/m3) for each block. While the speed of sound did not change dramatically with the density variations, the overall impedance could be greatly varied due to the wide density swing. For example, this could translate to rough impedance differences of between 0.80 and 0.91 on the MZ product. Due to the manner in which these enhanced density materials are made, the stratification between the top and bottom of a block can be even greater than the 2 lb/ft3 difference between individual blocks. Material cut from the bottom or top of a block may show these dramatic differences while the average block density is within specification. For MZ this may mean a 10 – 20% difference in both the speed of sound and impedance through the block thickness. Therefore it is crucial that the designer understands these variations while making material choices. There are ways to preselect and/or classify the products to minimize these variations if a very narrow impedance range is needed and we invite designers to have discussion as early as possible in their process.

Given these differences, we continued to explore the acoustic properties, but now we looked at them from two views. Our first goal was to measure the average properties of our high performance materials (lowest density per depth) over the established density and depth range. In this way we could determine the lowest attenuation levels for a given operating depth. Our second area of interest was in the measurement of our tooling materials (HYTAC®). We took this unusual step because the driving force behind the design of the tooling product line was not to achieve the lowest density possible. This allowed us much more flexibility in the manufacturing process allowing for a product that can be directly cast to different shapes and sizes, such as small (2” / 51mm) diameter rods. The cost advantage in utilizing a wider range of starting shapes is significant. Lower machining and material yield losses directly result in lower production costs for our customers. Further, these products do not have the same wide density variation as the enhanced density products offering better consistency in acoustic performance.

In the third and final installment, we will review material properties of our HYTAC line of syntactic foams to illustrate differences in matrix material.

AI-24 syntactic foam

Acoustic Properties of Syntactic Foam – Part 1

One of the great things about syntactic foam is that one can make an almost infinite amount of changes to the types and levels of the constituent materials in order to impart particular properties to the final part.  The types of matrix materials, hollow spheres, additives, and combinations thereof offer materials engineers a wide range of tools to determine end-properties.  Unfortunately, because the possibilities are nearly endless, it is difficult to stop making changes in order to perfect a particular property.  This holds especially true in the area of acoustic properties.  As part of our investigation into the development of products for industries interested in the acoustic properties of syntactic foams, we have discovered a number of interesting areas for exploration.

Acoustic Properties

When we think about acoustic properties, we generally think about whether a material absorbs or reflects sound.  For this discussion, we are thinking about syntactic foam as more of a window that sound waves can pass through.  In very general terms, the degree to which sound is reflected or absorbed by a material is dependent upon the acoustic impedance of the material and the media through which the sound is traveling (in the case of buoyancy materials, this is mainly water).  Other factors, such as wave frequency and incident angle can also play a role, but these are not within our control.  In most cases we are simply trying to match the acoustic impedance of water.  The acoustic impedance of seawater is ~1.50 MRayls[i] and is defined as the speed that sound can travel through the media (~1,450 m/sec) times the density (~1025 kg/m3).  These values will vary depending on things like temperature, water salinity and depth.  Variations can be as great as 10% or more.

Using 1.50 as an initial target, we set out to understand how close we could come with our standard syntactic products.  Measurements were performed on the different types of syntactic foams with a 1 MHz transducer.  Sample densities were also recorded.  Speed of sound and the calculated acoustic impedance in MRayls were reported for each material.  Overall speed of sound was found to be mainly dependent on density but was not independent of the modulus of the matrix material used.  Also, the variation in density through the thickness of the material was found to be very important.

In the next installment, we discuss comparative testing and data for multiple syntactic materials.

[i] MRayl, or Rayleigh, is a unit of measure used to describe characteristic acoustic impedance.

li-on batteries

Syntactic Foam & Li-on Batteries

With the recent ban on transporting lithium-ion batteries as cargo in passenger planes, regulators and industry are exploring novel ways to contain thermal runaway conditions. Syntactic foams offer potential due to their critical material properties.